On part one of the season four finale, Walter is forced to revisit his painful past.

The Fringe team encounters a nanites device implanted in a public escalator. It enters the bloodstream and kills upon movement.

They get one infected volunteer, Jessica, to work with them at the lap.

David Robert Jones is in our universe, and responsible for the nanites, but William Bell himself is pulling the strings of Jones.

Walter immediately recognizes Belly's work and tries to convince everyone else that he is alive.

William Bell sets Jones to take out the bishop in his chess game.

Cortexiphan is regenerative.

Olivia and Peter stop the beam of light bearing into earth, and Olivia uses her powers to stop Jones.

Jones dies.

Astrid and Walter set out in search of William Bell and happen upon the crates full of creatures we last saw floating on a ship.

Astrid is shot and William Bell walks up to his old friend, Walter Bishop.